Photolithography is a process for creating changes in a light-sensitive medium. In order to selectively expose portions of a light-sensitive medium using direct photolithography, a light-blocking medium, or mask, is placed between a light source and the light-sensitive medium. Alternately, a reflective mask may be used to reflect light on to the light-sensitive medium. When the features are large, the exposed shapes of the light-sensitive medium closely correspond to the shapes of the features of the mask. However, as feature size shrinks, behaviors of light such as diffraction, fringing and interference become more significant. Reducing the wavelength of the light source reduces these effects. In the production of semiconductors, photolithography is performed using ultraviolet light sources and other radiation beams such as ion beams, x-ray, extreme ultraviolet (EUV), and deep ultraviolet (DUV). When high-frequency light is not sufficient, a mask may require alterations such as serifs or hammerheads to expose a desired design feature. A mask may also require subresolution assist features, features that are not large enough to resolve directly on the light-sensitive medium but that are large enough to affect the behavior of light passing through other features. These alterations are termed optical proximity correction (OPC) features.
The same light behaviors that necessitate OPC features prevent mask features from being inspected directly. Instead, a simulation model is used to predict how a mask based on the design specification will appear upon inspection. This model can then be compared to an image of the mask. However, comparisons based on these models report a significant number of differences that do not correspond to actual mask errors. Frequently, these differences must be manually evaluated to determine whether mask corrections are required. Evaluation is expensive, laborious and time-consuming. Methods of increasing the accuracy of the models and reducing the number of false errors present a real value.